Season beef with cumin, oregano, salt and pepper. Place in slow cooker. Add tomatoes, beef stock, garlic, chipotles, adobo sauce and onion. Cook on low for 4-6 hours, until meat is tender and falls apart.

Remove meat from slow cooker; reserve a cup of liquid. Strain out the tomatoes, chiles and onions and use as a “salsa” topping later, if desired. Shred the beef and moisten with reserved liquid if needed. Serve beef.